The present invention relates to a method of removing binder material from a binder and particulate mixture and more particularly relates to a method of removing binder material from shaped articles under vacuum pressure conditions.
It is well known in the art to form shaped articles of particulate material by injecting a heated particulate and binder mixture into a mold in such a manner that the resulting product retains the shape of the mold upon cooling. Subsequent to ejection from the mold, the shaped mass is sintered to bond the particles together and thereby provide the physical characteristics and stability necessary for the article's intended environment of use. Because the binder material generally consists of low melting point hydrocarbon-based materials, typically waxes, plastics and polymers, the binder material often volatilizes or decomposes prior to sintering into substances which can chemically react and combine with the particulate particles. The effect of such a reaction on the chemical and physical properties of the sintered product is particularly deleterious when the particulate is a metal or metal alloy and the decomposition product contains carbon.
In order to overcome the problems caused by the thermal dissociation of the binder, the prior art has developed several methods for debindering the shaped article prior to the sintering process. Solvent extraction of the binder material is disclosed in U.S. Pat. No. 2,939,199 to Strivens whereby the extraction is carried out by immersing the shaped part in either boiling solvent, hot solvent or solvent vapor. Because of the high temperatures involved in the extraction process, temperature gradients form in the shaped part which can lead to the cracking or fracture of the desired product. Another problem is that solvent extraction techniques can pose a health risk to the employee operating the process. Furthermore, national or state pollution laws may require recovery or treatment of the solvents utilized which can add substantial cost to the binder removal process.
In addition to removing binder materials by solvent extraction, Strivens also disclosed a method of debindering shaped articles through vacuum distillation. The binder is first transformed to its liquid state by melting whereupon it is then removed from the article surface by evaporation. Transformation of the binder from the solid phase to the liquid phase and from the liquid phase to the vapor phase at the surface of the article can lead to distortion of the part being manufactured to such an extent that the required tolerances for the desired application cannot be met.
A method for evaporating binder material from a green body by blowing a non-saturated chemically inert gas or air at atmospheric pressure over the product surface on which liquid binder is present is disclosed in U.S. Pat. No. 4,404,166 to Wiech, Jr. As normally practiced, the flowing gas is air which is known to react and combine with particulate metal and metal materials to produce chemical oxides which can adversely affect the final physical and chemical properties and characteristics of the products. In order to ensure the continuous removal of the liquid binder from the wet surface, the atmosphere adjacent to and in contact with the green body surface must always be maintained in an unsaturated condition. If the flow of the chemically inert atmosphere or air is in any way disturbed or impeded, which can commonly occur during commercial operation, the efficiency of the removal of the binder from the shaped article will be seriously impaired. An additional limitation is that both the expansion and outward migration of the liquid binder from the interior to the surface of the green body generate internal pressure forces which can lead to cracking or distortion of the shaped article.
A method for continuously evaporating binder material from a shaped article is disclosed in U.S. Pat. No. 4,305,756 to Wiech, Jr. The article to be debindered is held in a closed chamber at a pressure many orders of magnitude greater than the vapor pressure of the binder material at the ambient temperature within the chamber. Under these processing parameters, the binder transforms first to the liquid phase and from the liquid phase the binder material is then evaporated into the furnace atmosphere. To enable the continuous distillation of the liquid binder from the green body, Wiech provides for the condensing of the binder vapor onto a cold collecting region. The continuous condensation of binder vapor at the cold region in the furnace creates a driving force for continuing evaporation of the liquid binder. An uncontrolled or nonuniform removal rate will cause the formation of internal pressure gradients in the green body which can lead to cracking or rupture of the shaped article. An additional limitation of this process is that it is very time consuming, requiring at least 12 hours for the removal of a simple paraffin binder.
The sublimation of the first of two binder materials from a powdered metal and binder mixture prior to sintering is disclosed in U.S. Pat. No. 4,225,345 to Adee et al. Removal of the first binder material, which is camphor, takes place at room temperature and at a partial pressure of 10 inches of mercury. Removal of the second binder material, which is either polystyrene or paraffin, is disclosed only as being removed by solvent extraction or thermal decomposition.
The removal of binder material from a shaped article by sublimation is also disclosed in U.S. Pat. No. 3,769,044 to Horton. The two phase binder system consists of a first binder atmospheric pressures, such as camphor or paradichlorobenzene, or which requires slight heating to facilitate sublimation such as anthracene or benzolic acid, and a second binder which is nonsublimable at these temperature and pressure parameters. Sublimation of the sublimable binder is facilitated, if necessary, by subjecting the article to a partial pressure of approximately 27 inches of mercury. Because of the low vacuum or partial pressure conditions which are required, only those binders which naturally sublime at or near room temperature and atmospheric conditions can be utilized. In addition, the Horton process is inordinately time consuming requiring from four to ten hours to remove the camphor or paradichlorobenzene.
Hence, the prior art still lacks a method for removing binder material from a binder and particulate mixture which proceeds at an enhanced rate, does not require formation of a liquid binder phase on the mixture surface which can adversely affect part integrity and which will effectively remove binder materials that are not readily sublimable at room temperature and pressures greater than 1 Torr.